I guess my question is: How long is this survival situation going to last?

During stressfull times people get all kinds of digestive disturbances, from nausea and vomiting to constipation and diarrhea. I have eaten nothing but MREs for about 6 days straight when backpacking and kept myself pretty regular (pardon the details <img src="/images/graemlins/smile.gif" alt="" />), while other members of my group had all kinds of issues.
As Chris pointed out, try eating your survival food for a "reasonable" amount of time you would be in a survival situation, and see how your body reacts.
Although fiber is good for you, its also on the heavy/bulky side, requires water to be effective, and tends to spill all over your gear if that ziplock rips. Try something like Senokot pills if you develop a problem with your survival food.
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